Add support for ghc-7.10 registration. User visible changes: ghc-package stopped exporting pkg_* phases and now they are reexported by haskell-cabal. pkg_* phases do not install any additional files anymore.

Added support for CABAL_EXTRA_BUILD_FLAGS and HCFLAGS magic variables
CABAL_EXTRA_BUILD_FLAGS allows user to pass arguments to ./setup build phase.
For example you can get detailed build log this way:
CABAL_EXTRA_BUILD_FLAGS=-v emerge something
HCFLAGS is similar to CFLAGS / CXXFLAGS / FFLAGS in spirit.
You can use it in make.conf or in one-shot merges to enable
random ghc features:
HCFLAGS="-O0 -Wall" emerge something-else

Updating the haskell-cabal eclass to support the CABAL_FEATURES nocabaldep
which is used in the dev-haskell/haskell-updater package. It enables
packages to use the cabal eclass without depending on cabal.

Ask portage rather than ghc-pkg about which version of cabal to use.
This way we build packages using version of cabal from portage that we
tested rather than a version that the user installed. This is important
as many packages are sensitive to changes in the Cabal API.
Make ghc-getghc and ghc-getghcpkg functions return absolute paths and fix
up one place in the haskell-cabal eclass where this had a knock-on effect
this should fix bug #140589

new eclass for Haskell packages that make use of the Common Architecture for Building Applications and Libraries (cabal)

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